After working in a variety of secondary roles, he got his big break in 1968 when he was given a lead role next to Martin Milner as rookie LAPD police officer James A. "Jim" Reed on Adam-12, a police drama television series created by Jack Webb. The popular show ran on NBC from 1968 to 1975 and made McCord a household name. In the first season of Jack Webb's Dragnet 1967, he appeared twice. The first appearance, in the episode "The Big Explosion", saw him credited under his legal name.[1] The second, in the episode "The Interrogation", saw McCord credited under his stage name and he played the role of an LAPD officer that had allegedly been caught in a burglary ring. McCord went on to appear five additional times in the second season, three times as unnamed officers and twice as his eventual Adam-12 character Jim Reed. He appeared one more time on Dragnet as Reed before the character became exclusive to Adam-12.

In 1972, McCord was elected to the National Board of Directors of the Screen Actors Guild and served on the Board for 11 years. He served as 1st National Vice President[2] while on the National Board of Directors.[3]

In 1980, McCord played the lead character Troy on the short-lived series Galactica 1980. Two years later, Jack Webb tabbed him for a new Dragnet series he was ready to launch, with McCord to play the partner to Webb's Joe Friday. It was never made clear if Webb was intending to have McCord play a new role or to simply revive his Adam-12 character Jim Reed, as Webb died before any of the episodes he wrote could be produced. Also in 1982, McCord had a role as Dave Unger, a navigator on the "Lunar Shuttle" Mayflower in that year's comedy Airplane II: The Sequel, McCord's only comedy role. McCord re-teamed with Martin Milner, his co-star in "Adam-12", in the cable TV-movie Nashville Beat (1990), originally shown on the now-defunct The Nashville Network (now Spike). In this story that McCord partially wrote, he played an LAPD detective who teams up with his old partner (Milner) in Nashville. In 1990 McCord appeared in the film Predator 2 as Captain Pilgrim. In 1991, he appeared in the film Out for Justice as Jack. From 1994 through 1995, McCord played the recurring role of Scott Keller on seaQuest DSV. McCord appeared as a downed fighter ace on Baa Baa Black Sheep in the 70s.

In 1999 he was introduced to a new generation of fans as a semi-regular guest star on the science fiction television cult hit, Farscape. He played two versions (one human and one alien appearing in the physical form of the human) of the same character, Jack Crichton. In 2003, he ran for President of the Screen Actors Guild but lost to Melissa Gilbert by a margin of 42% to her 50%.

McCord appeared in three episodes of the CBS television military courtroom drama JAG, two of them as a naval court judge. He guest starred in season 1 episode 5, "Presumed Dead", of the television series Baa Baa Black Sheep as Capt. Charles W. Dobson.